Modem subscriber television systems (STS) transmit digital content, which is packetized, from a headend to a subscriber. The digital content is typically provided in a format such as MPEG or in other packet formats known to those skilled in the art. An operator of an STS typically prefers to provide programs in digital format because digital programs provide superior fidelity and because digital programs are compressed so that they generally use less bandwidth than analog programs. Digital programs are compressed using, in part, a quantization parameter.
Frequently, the operator of an STS may want to convert a compressed digital signal of a given bit rate into a compressed digital signal of a lower bit rate by using a conventional transcoder to change the quantization parameter. A conventional transcoder used for such a purpose consists of a cascaded decoder and encoder. This combination is rather complex and expensive. In the particular case of video signals, some other aspects have to be taken into account. A coded video signal consists of a succession of encoded video-frames, where each video-frame is subdivided into a two-dimensional array of macroblocks, each macroblock being composed of blocks. A video-frame may be in the spatial domain, which is the pixel domain, and is transmitted in the frequency or transform domain, which results from a Discrete Cosine Transform (DCT) of the video-frame in the spatial domain. In addition, a video-frame may be separated into two fields: the top field formed by the odd lines of the video-frame and the bottom field formed by the even lines of the video-frame. A macroblock may be conveyed in two different formats: an interlaced format and a de-interlaced format. In the interlaced video-frame format, a macroblock is composed of lines from the two fields alternately and each DCT-block of the macroblock is formed by data from the two fields. In the de-interlaced format, a macroblock is composed of lines from the two fields, and each DCT-block of the macroblock is formed by data from only one of the two fields. Each DCT-block of a video-frame is scanned and encoded.
Before a conventional pixel-domain transcoder can requantize a bit stream, the decoder portion of the transcoder converts the bit stream into pixel domain values. The encoder portion of the transcoder then requantizes and converts the pixel domain values back into DCT-domain values.
In addition to conventional pixel-domain transcoders, there exist conventional DCT-block domain transcoders, which operate in the DCT-block domain. Such a transcoder receives a bit stream and converts the bit stream into sets of run-level pairs, where a set of run-level pairs is a compressed representation of a DCT-block, and then converts the sets of run-level pairs into DCT-blocks. The transcoder manipulates information in the DCT-block domain and then reconverts the DCT-blocks back into sets of run-level pairs, which are then converted back into a compressed bit stream. Further details regarding DCT-block domain transcoders can be found in “A Frequency-Domain Transcoder For Dynamic Bit-Rate Reduction of MPEG-2 Bit Streams,” Assuncao et. al., IEEE Transactions on Circuits and Systems for Video Technology, Vol. 8, Issue 8, December 1998, pages 953-967, which is hereby incorporated by reference in its entirety; and “Manipulation and Compositing of MC-DCT Compressed Video,” Chang et al., IEEE Journal on Selected Areas In Communications, Vol. 13, No. 1, 1995, pages 1-11, which is hereby incorporated by reference in its entirety.
Both the pixel-domain and DCT-block domain transcoders operate on full 8×8 blocks of data. In the DCT-domain the DCT blocks are sparsely populated, since most of the elements in the block are zero. Typically, no more than six of the sixty-four elements in the block have a non-zero value. Therefore, what it is sought is a transcoder that operates in the run-level domain because of the large computational savings that result from operating the transcoder in the more compressed run-level domain than in the expanded pixel or DCT-block domains and that can compress a frame of information to an approximate desired size.